Limonene is a monoterpene that is present in orange peel oil and has been reported to have activity against mammary (Elegbede, et al., 1984, Carcinogenesis 5:661-664; Elegbede, et al., 1986, J. Natl. Cancer Inst. 76:323-325; Haag, et al., 1991, Proc. Am. Assoc. Cancer Res. 32:402; Elson, et al., 1988, Carcinogenesis 9:331-332; Maltzman, et al., 1989, Carcinogenesis 10:781-783), lung, and stomach (Wattenberg, et al., 1991, Carcinogenesis 12:115-117) cancers. Limonene has also been shown to inhibit certain skin tumors. Van Duuren, et al., 1976, J. Natl. Cancer Inst., 56:1237-1242; F. Homburger et al., 1971, Oncology, 25:1-20.
Although studies have shown that limonene is not toxic to humans at the required usage levels, treatment with limonene is not without some side-effects, particularly when a large dose of limonene is required in a short period.